Patent Document 1—Japanese Patent No. 3771081, Application No. 11-086460.
Patent Document 2—Japanese Patent Laid-Open No. 2005-50349.
Patent Document 3—Japanese Patent Laid-Open No. 2004-246921.
Patent Document 1 discloses a non-mechanical or optical mouse and a pointing device that are currently commercially available. Such a device emits light and illuminates an operation plane at a small angle of incidence, 70° to 85°, and detects fine texture of the surface based on highlighted and shaded portions. An LED is generally used to produce light, while in some cases, a laser diode (VCSEL, in particular) is used to improve the performance of identifying the fine texture.
Patent Document 2 describes a pointing device including an optical motion detection circuit that determines the amount of movement by emitting infrared light and detecting correlation between images based on the reflected light from a surface. The motion detection circuit (pointing device) correlates a reference frame formed of digitized output values of the reflected light obtained from an optical detector with a sample frame formed of digitized output values obtained subsequent to the reference frame so as to check the motion in a predetermined direction.
A known pointing device currently commercially available is, for example, a motion detection device described in Patent Document 3. This device includes a light source that produces coherent light, such as a laser diode, and a navigation sensor. The device detects reflected light of the light from the light source to determine the amount of movement of the pointing device (a mouse, for example).
The pointing device described in Patent Document 2 or Patent Document 3 may not correctly operate or may malfunction when used on a surface made of glass material. This results from the fact that an equivalent of the fine texture detectable in Patent Documents 2 and 3 will not be detected because the glass material has extremely few surface irregularities. Furthermore, the fact that surface roughness of the glass surface, measuring about a few nanometers, is much smaller than not only the wavelength of the incident light but also each pixel in the optical detector also contributes to the difficulty in detecting fine texture.
The signal intensity of the reflected light described above will now be briefly explained. In general, to detect the intensity of light reflected from a substrate surface having certain surface roughness as grayscale variation, the size of the grayscale variation needs to be larger than the wavelength of the light and substantially the same as the size of a pixel used for detection.
Since the surface roughness of a glass material is extremely small, that is, about a few nanometers, it is difficult to detect the amount of change in intensity of the light reflected from such a surface with high sensitivity.
In specular reflection, the intensity of reflected light is basically equal to the intensity of incident light, but varies according to the reflection coefficient of the surface when its material is glass or the like (the reflection coefficient depends on the polarization direction and the angle of incidence of the light). Letting R be the reflection coefficient and IIN be the intensity of the incident light, the average intensity IR of the reflected light is expressed by the following equation:IR=R×IIN   (Equation 1)In general, the average intensity Id of speckle light emitted from the surface is expressed by the following equation:Id≅(4πσ/λ)2×IR   (Equation 2)
There is a need for a pointing device, such as an optical mouse, that can be used on a smooth substrate made of glass or the like.